A few months ago, Google open sourced the VP8 video codec as part of the WebM video project, to create a truly Free/free unencumbered video format for the web as an answer to the non-Free/free patent-encumbered H264 format. Today, Google launched a new image format for the web, WebP, which aims to significantly reduce the file size of photos and images on the web.

Realistically thought compressing already processed jpegs is probably the most likely use case for the web. I have 20 thousand or so jpegs in my htdocs folder right now. If I can run a script over them and re-compress them in a new format and get smaller files without too much loss of quality them I'm totally interested. If on the other hand I have to go back to the unprocessed original and start from to get any benefits, then I kind of lost interest.

You don't get the point: The mission of image compression is to reduce the size of the image with a minimal loss of information.
You may talk about different kinds of information loss, and you may prefer some kinds of loss over other kinds of loss. But you cannot talk about improvement. Any change of the image caused by the compression technique - whether you find it improving or not - is a change, and thus it is bad.
If you want to "improve" an image, you use other techniques (sharpening, blurring, scratch detection, etc.). And then you get a new "original" that you may try to compress with different compression techniques.